1. Field of the Invention
The present invention releases to an apparatus for demodulating an information signal and, more particularly, to a demodulation apparatus suitable for demodulating a signal modulated by spread spectrum modulation and a communication system using the same.
2. Related Background Art
A spread spectrum communication is a communication for transmitting an information signal by spreading the information signal to a sufficiently wide bandwidth, and has the following features. That is, the spread spectrum communication allows code divided multiplexing, discriminates strongly against a disturbance, has a high privacy function, and so on.
In reception processing of the spread spectrum communication, an information signal is demodulated by executing so-called inverse spreading processing for correlating spread codes assigned to respective channels with a received signal. Conventionally, a receiver of the spread spectrum communication is divided into a synchronization unit and a demodulation unit. The synchronization unit achieves synchronization by a sliding correlation method for detecting a correlation between received spread codes and reference spread codes having a bit rate slightly different from that of the received spread codes.
As a system capable of high-speed synchronous acquisition, a system using a convolver is known. The convolver is a convolution arithmetic element, and serves as a correlator when one of two input signals is set to be a temporally inverted signal. More specifically, when a received signal is input as one of input signals to the convolver, and a signal obtained by temporally inverting the received signal is input as the other input signal, the two signals coincide with each other at a certain timing, and generate a sharp peak output. In particular, if spread codes used in this case are those with good auto-correlation characteristics, a sharp peak output is generated only when the two signals coincide with each other; otherwise, almost no output appears. As one of convolvers, an elastic surface wave convolver is known. The elastic surface wave convolver is effective for high-speed transmission since it is an analog arithmetic element, and can execute signal processing in real time.
FIG. 1 shows a conventional spread spectrum receiver using an elastic surface wave convolver. Referring to FIG. 1, the receiver comprises first and second frequency converters 2 and 4, an elastic surface wave element (convolver) 5, a filter (F) 6, a detector (D) 10, a peak detection circuit (PD) 8, and code generators (CG) 3 and 9. A received signal 1 is converted into an intermediate frequency by the frequency converter 2, and is input to the elastic surface wave element 5. The code generator 3 generates codes obtained by temporally inverting spread codes of the received signal as reference spread codes, and inputs these codes to the elastic surface wave element 5 via the frequency converter 4.
The elastic surface wave element 5 comprises first and second excitation electrodes 102 and 103 for exciting an elastic surface wave on a piezoelectric substrate 101, and a rectangular output electrode 105 arranged between the first and second excitation electrodes 102 and 103.
Each of the first and second frequency converters 2 and 4 comprises an oscillator 15, a multiplier 13, and a filter (F) 14.
When the received signal 1 is input to the first excitation electrode 102, and the output from the code generator 3 is input to the second excitation electrode 103, a first elastic surface wave excited by the first excitation electrode 102 and a second elastic surface wave excited by the second excitation electrode 103 overlap each other on the output electrode 105 while propagating in opposite directions. Since the displacement and potential of a product of the two elastic surface waves are generated on the substrate 101 to have a frequency twice that of the input signal and a wave number=0 by the parametric mixing phenomenon of the two overlapping elastic surface waves, the output electrode 105 can extract the overlapping elastic surface waves as an electrical signal by integrating them within the range of the output electrode. Therefore, the elastic surface wave element 5 generates a sharp peak output at a center frequency 2f (where f is the center frequency of the input signal) when the two elastic surface waves coincide with each other. This output is extracted via the filter 6, and is envelope-detected by the detector 10. Thereafter, the output from the detector 10 is subjected to peak detection by the peak detection circuit 8. On the basis of peak information obtained by the peak detection circuit 8, the generation timing of the reference spread codes to be generated by the code generator 3 is adjusted, so that the reference spread codes generated by the code generator 3 coincide with the received signal on the elastic surface wave element 5 in a desired state, thus synchronizing the codes.
The above-mentioned peak information is also input to the code generator 9 for generating the same spread codes as those of the received signal. When the code generator 9 generates spread codes in synchronism with the received signal, and a multiplier 11 multiplies the generated codes with the received signal, the received signal modulated by spread spectrum modulation is inversely spread. Since the inversely spread signal is a modulated signal modulated by frequency modulation, phase modulation, or the like, which is normally used, the signal is demodulated via a conventional demodulator (DM) 12.
However, in the prior art described above, the output from the elastic surface wave element is used only for synchronizing codes, and the demodulation unit must be arranged in addition to the synchronization unit, resulting in a large circuit scale.
On the other hand, Nakagawa et al. "dc effects in elastic surface waves", Applied Physics Letters, Vol. 24, No. 4, pp. 160-162, 15 Feb. 1974 discloses an elastic surface wave element shown in FIG. 2. Referring to FIG. 2, input interdigital transducers 17 and 18, and an output interdigital transducer 19 are formed on a piezoelectric substrate 16. When a pulse signal having a wave number=k and a frequency=.omega. is input to the transducer 17, and a pulse signal having a wave number=-k and a frequency=.omega. is input to the transducer 18, the two transducers 17 and 18 generate elastic surface waves which propagate in opposite directions. These elastic surface waves cause an interaction on a region between the transducers 17 and 18, and a signal having a wave number=2k and a frequency=0 is extracted from the output transducer 19 by a nonlinear effect.